Not a van but my family had fish and chip shops. There is a lot of hidden work, from chipping the potatoes onwards. Most places have massive chippers as it's not something you can do manually. Then you need huge storage bins for the chipped potatoes, as they need to be kept in water or they discolour, as well as large sinks as you need to take the eyes out or what the chipper missed.
It's unlikely uou could do the prep in a domestic kitchen, the storage for the potatoes alone is quite substantial. Then you need huge fridges for the rest of the food. Fish, chicken, pizza, sausage or whatever you're serving. And then on top of this there is other prep, such as making the batter etc, through to cleaning the fryers.
You also need storage for other things you may wish to sell, ie soft drinks or whatever.
So as a pp said, you'd need to do the prep work before effectively going out to sell.
For my family the day started at the latest by 10 am. Then the shop opened late afternoon, and then finish was 11 pm, after which was clean up.
It can be very profitable, but it's a lot of work with long days. You'd need a fair chunk of money behind you to get yourself set up.